There are image reading devices that include a sheet conveyance device to separate and transport originals placed on a document table one at a time consecutively and an image reading unit to read images of the originals. In such image reading devices, it is preferred to reduce intervals between the originals (hereinafter also “sheets”) to shorten reading time. Similarly, in image forming apparatuses that form images on sheets that are transported consecutively from a sheet tray one at a time, it is preferred to reduce intervals between sheets to shorten printing time.
Therefore, detectors (hereinafter “trailing-edge detectors”) are used to ascertain when the sheet conveyed from the document table or sheet tray exits from the separator. However, it is difficult to timely recognize the timing at which the sheet exits from the separator from the following factors. The separator separates a single sheet from a bundle of sheets placed on the document table to prevent multiple sheets from being transported together at a time, which is a phenomenon generally called “multifeed”. When multiple sheets enter a separating position with their leading-end portions superimposed one on another, the separator returns the subsequent sheets to the document table while transporting only the top sheet in a sheet conveyance direction. Thus, only the top sheet can be discharged by the separator. To minimize intervals between sheets, it is preferred that the subsequent sheet be sent out immediately after the trailing edge of the preceding sheet exits from the separator. More specifically, the separator typically includes a belt-shaped or roller-shaped conveyance member to transport the sheet in the sheet conveyance direction and a reverse roller pressed against the conveyance member, thus together forming a separation nip. The top sheet is separated from the rest in the separation nip, and the reverse roller returns the rest to the document table. Above and beneath the downstream end (i.e., exit) of the separation nip in the sheet conveyance direction, curved surfaces of the conveyance member and the reverse roller, projecting in the sheet conveyance direction, are present. In this configuration, the trailing-edge detector is disposed at a distance from the exit of the separation nip not at the same position as the exit of the separation nip. Consequently, it is inevitable that feeding the subsequent sheet is triggered by detection of the trailing edge of the preceding sheet that has been transported a given distance from the separation nip, inhibiting reduction in intervals between sheets.
Intervals between sheets may be reduced by increasing a conveyance velocity at which sheets are transported from the document table or sheet tray to the reading position or image forming position from a velocity at which the sheet passes through the image reading position or image formation position. Although this approach is effective to some extent, image reading velocity or image formation velocity has become faster owing to progress in technology, and it is difficult to increase the conveyance velocity as described above.